Voice communication is evolving from circuit-switched technology, such as provided by the Public Switched Telephone Network (PSTN) or Public Land Mobile Network (PLMN), to packet-switched technology, such as provided by Voice over IP (VoIP) techniques across IP Networks. Indeed, the Internet Engineering Task Force (IETF) has developed IP-based protocols to perform various functions in VoIP communications. In particular, for example, Session Initiation Protocol (SIP), specified in IETF request for comment document RFC 2543, has been developed for establishing voice calls between two parties. In contrast, Real-time Transport Protocol (RTP), specified in IETF request for comment document RFC 1889, has been developed to format packetized voice to be carried over the Internet once the call has been established.
At the same time as voice communication is evolving, wireless networks are evolving from circuit-switched voice networks (e.g., GSM, IS-136, IS-95) to packet-switched networks (e.g., WLAN, UMTS, cdma2000) capable of supporting multimedia applications to mobile end-users over IP. General Packet Radio Service (GPRS), which is an evolution of GSM, can support packet data (e.g., web browsing, email) in a cellular environment. Further evolution of GPRS, often referred to as the Universal Mobile Telecommunication System (UMTS), is expected to support real-time multimedia over IP (e.g., VoIP, video over IP, streaming media) in a cellular environment. In addition, the Third Generation Partnership Project (3GPP) has specified the IP Multimedia Subsystem (IMS) in UMTS to accomplish the control and service functions of wireless IP multimedia. In this regard, the 3GPP has adopted SIP as the signaling protocol in IMS. At the same time, in the cdma2000 world, the 3GPP2 has been developing the IP Multimedia Domain (MMD) to implement the control and service functions of wireless IP multimedia. The 3GPP2 has also adopted SIP into the MMD specification.
Given this evolution of wireless communications systems, the modern communications industry has brought about a tremendous expansion of wireline and wireless networks. Currently, a given mobile terminal may operate in more than one type of wireless network such as a WLAN as well as a cellular network. However, the presence of multiple wireless technologies in a communications system poses new problems and presents new opportunities for an operator or service provider to utilize and manage resources across these technologies. Additionally, mobile terminals operating in two different communications systems (i.e., dual mode) such as WLAN and cellular network may not be as efficient as desired in conserving power.
The Institute of Electrical and Electronics Engineering (IEEE) has defined a set of specifications for WLAN as 802.11x, where x denotes different specification groups. IEEE has defined three main methods for power saving in a WLAN, namely, PS-Poll, Scheduled Automatic Power Save Delivery (APSD) and unscheduled APSD. However, IEEE apparently defined the power save methods without considering the mobility aspect of a mobile terminal. Since 802.11 is primarily focused in providing wireless access to terminals such as laptop computers. Each of the three methods will be described below in turn:
PS-Poll Power Save: This technique is a basic polling based power save method specified in 802.11b. A station (STA) goes into “Doze” mode for a “ListenInterval” (multiple of the beacon transmission period), after indicating to the Access Point (AP) using a “Power Management” bit in the MAC header. When the AP has a Media Access Controller (MAC) Service Data Unit (MSDU) to deliver for the STA, it sends a Traffic Indication Map (TIM) for the STA. The STA wakes up after its “ListenInterval”, and listens to the beacon. If it sees a TIM for itself, it sends PS-Poll to the AP. The AP, in turn, transfers data to the STA. If the STA does not see a TIM, it goes back to the “Doze” mode for another “ListenInterval”.
Scheduled APSD: This method is specified in 802.11e. In this method, an AP only transmits the MSDU in a scheduled interval. The scheduled interval is set between AP and STA by exchanging a service interval and service start time. A STA only wakes up at the scheduled interval. 802.11e provides two QoS mechanisms: Enhanced Distributed Channel Access (EDCA) and Hybrid Control Channel Access (HCCA). EDCA is a contention-based mechanism. HCCA is a controlled based access mechanism. The scheduling works best on top of HCCA. Scheduling is difficult to achieve with the EDCA, due to its inherent contention based nature. This method is more suitable for HCCA.
Unscheduled APSD: This method is also specified in 802.11e. In this method, there is no prior scheduling setup between AP and STA. An AP indicates the traffic to the STA, similar to the PS-Poll method. The STA can wake up at any time and send a frame to the AP. When the AP receives any frame from the STA, it starts delivering MSDUs.
The above-described power save methods of a WLAN may be deficient for a mobile wearable device (e.g. voice or multi-media terminal) which changes APs quite frequently. For example, a WLAN has no dormancy capability, and thus, whenever the mobile terminal goes out of the coverage area of its current AP, the mobile terminal continuously tries to identify a beacon corresponding to a new AP. After finding a beacon, the mobile terminal performs WLAN Re-association with the new AP. These procedures drain the power resources (e.g. battery) in the mobile terminal. As a consequence, the battery life of the mobile terminal may be compromised resulting in dropped calls or more frequent charging of the battery by the end user which are often annoying to the end user.
In light of the shortcomings described above, there exists a need to provide a mobile terminal operating alternatively in a cellular network and a wireless network such as a WLAN (for example) with power consumption comparable to a mobile terminal operating only in a cellular network since such power consumption will be in line with the expectations of the users. It is readily apparent to one skilled in the art that the sleep mode in a cellular network (i.e., Paging Mode) is much more power efficient than the sleep mode (i.e., Doze Mode) in WLAN. As such, it would be advantageous for a dual mode mobile terminal capable of operating in a cellular network as well as a WLAN to utilize the cellular network interface in its sleep mode. However, a local area network, for example a WLAN network, owned by an enterprise, typically requires a mobile terminal operating in a dual mode to be primarily reached by their address in the WLAN network such that the WLAN network, not cellular network, is used while the terminal is in the WLAN coverage area. This requirement helps in reducing cellular operator charges and also helps in monitoring user activity. However, there are currently no techniques available for delivering/initiating call/session in a WLAN, while the mobile terminal is in the cellular sleep mode (i.e., no procedure for waking up a cellular dormant terminal in the WLAN). As such, the power savings afforded by cellular sleep mode cannot generally be taken advantage of while in a WLAN.